青藏高原东北部多尺度重力场及其地球动力学意义

由于受到SN和EW双向挤压作用的综合影响,处于年轻高原与古老地块交接区的青藏高原东北部岩石圈强烈变形,构造活动十分活跃.为了整体性地了解青藏高原东北部重力场和深部动力学机制,本文基于EGM2008全球重力场模型数据,利用小波多尺度分析获得不同尺度的重力异常信息;同时反演了研究区的地壳厚度;通过构建穿越龙门山造山带和西秦岭造山带两条剖面的岩石圈密度结构模型,分析了地壳上地幔内不同介质的分布特征.研究结果表明,青藏高原东北部岩石圈显示十分复杂的塑性体的特征,其重力异常走向多以EW或SSE为主,反映了高原岩石圈物质向东运移的趋势;地壳厚度由西向东逐渐减薄,边缘造山带深部并没有发现"山根"痕迹,结合该地区低重力的特征,推测西秦岭—松潘构造结岩石圈发生过大规模地幔流底侵作用;地幔流上涌的动力可能来源于印度板块向欧亚大陆板块俯冲,激发了地幔流体侧向移动,在扬子地台和华北地台附近受到坚硬岩石圈的阻挡而被迫上移,并因此造成龙门山与西秦岭的隆升.

Multi-scale analysis to the gravity field of the northeastern Tibetan plateau and its geodynamic implications

Affected by both N-S and E-W directed compression,the lithosphere of the northeastern Tibetan plateau, located in the junction region of the youngest plateau and ancient blocks,has experienced active tectonic motions and intense deformation. Such dynamic processes should have left signatures in the gravity field of this region. To understand the gravity field and its relation to deep structure of the northeastern Tibetan plateau,this work made a multi-scale wavelet to the data from the Earth Gravitation Model 2008 (EGM2008) of this region. Our analysis separated the Bouguer gravity anomalies into different parts of shallow to deep and then estimated the corresponding source depths by logarithmic power spectrum technique. Moreover,we calculated the crust thickness and described the Moho topography. The two lithosphere density models respectively across the Longmen Shan orogenic belt and the western Qinling orogenic belt have been constructed, which permit to reveal the distribution features of the different materials in the crust and uppermost mantle beneath the study area. #br#The results show that the crust-mantle structure in northeastern Tibetan plateau is different from that of the peripheral areas. The Sichuan basin,the Ordos basin and the Qilian block have homogeneous and rigid crust-mantle structure on the whole. While the crust-mantle materials in the Tibetan plateau are heterogeneous and plastic,where gravity anomalies strike dominantly in EW and SSE. It is inferred that the lithosphere materials extrude towards east in the interior of the Tibetan plateau,and then turn to south due to resistance of the Yangtze and the North China platforms in the margin of the plateau. The image of the crust thickness indicates that the northeastern Tibetan plateau gradually thins from west to east. However,there is no trace of "mountain roots" under the peripheral orogenic belts. Instead,the Moho below these areas uplifts. Combined with the low gravity anomalies,this study suggests that the upwelling mantle heat-flow interacts with the substances in upmost mantle and lower crust,uplifting the Moho or forming the new shallow Moho.#br#We infer that under the influence of the northward motion of the Indian plate,thermal activity may have occurred in the Qiangtang block lithosphere,and then induced thermal mantle flow. This mantle flow shifts toward east along the asthenospheric channel. Due to the obstruction of the rigid lithosphere in the Yangtze platform and the North China platform, the mantle flow is forced to rise up in the western Qinling-Songpan tectonic node. The uplift of the Longmen Shan and the western Qinling as well as the generation mechanism of earthquakes in the western Sichuan and Gansu province may be all related to this dynamic process.